Abstract: Environment and Health Group, Tufts HIV/AIDS Clinic, and the Boston-area PrEP Community of Practice will collaborate under the leadership of Patricia Weitzman, PhD, to develop and test the feasibility and short-term effectiveness of the first-ever individually-tailored, developmentally- and culturally-sensitive, mobile (mhealth) PrEP adherence tool. It will target culturally-diverse, young adult men ages 20-29 who have sex with men (YMSM). When PrEP is taken daily, it can reduce HIV risk by up to 92%. The CDC estimates PrEP is appropriate for 500,000 MSM in the U.S. Impact models show even modest PrEP adherence could reduce new HIV infections among MSM by 29% over 20 years. The recent rise in HIV infection among young adults age 20-29, particularly minority YMSM, points to the value of PrEP uptake and adherence support for culturally-diverse YMSM. In PrEP trials, younger age was the most consistent correlate with low PrEP adherence. Extensive mobile phone penetration among young adults makes mhealth an ideal, low-cost method for providing PrEP adherence support to YMSM. We bring together a developmental understanding of young adult decision making with principles from social learning theory and positive psychology for supporting PrEP adherence. The proposed project is based on Dr. Weitzman's successful Phase 1 (and current Phase 2) anti-retroviral therapy (ART) mhealth adherence intervention for HIV+ African Americans, which combined tailored pill reminders with age- and culturally-sensitive motivational and educational texts. Experts suggest building PrEP adherence interventions on proven ART adherence interventions as many PrEP adherence facilitators are the same as ART. Moreover, the growing literature on mhealth shows it is a powerful approach to supporting adherence across conditions. In addition to building on our prior ART mhealth tool, we address facilitators/barriers identified in PrEP RCT follow up studies, and CDC PrEP adherence recommendations. Moreover, formative focus groups with diverse YMSM will allow us to identify and target unique adherence barriers and facilitators in this group. The intervention will provide tailored pill reminders with PrEP education texts, as well as developmentally-sensitive motivational texts. We will include sleep hygiene and alcohol use messages, which have direct relevance to young adults. By using a low-cost, accessible mobile phone tool, the intervention eliminates several access barriers to support, which can be a particular problem for low income and minority patients. Our overarching goal is to help stop the rise in HIV among YMSM.